In recent years, it is desired to reduce exhaust gas emitted from a vehicular internal combustion engine. Specifically, particulate matters such as soot and SOF are contained in exhaust gas of a diesel engine, as well as CO, HC, and NOx, and such particulate matters need to be removed from exhaust gas. Therefore, a particulate filter is provided to an exhaust passage to correct particulate matters in exhaust gas.
The particulate filter includes porous partition walls, through which exhaust gas passes. Particulate matters contained in exhaust gas are corrected when exhaust gas flows through the surface of the partition walls and pores. When the amount of particulate matters accumulating on the filter excessively increases, flow resistance of exhaust gas increases in the particulate filter. As a result, backpressure increases in the engine, and engine performance may decrease. Accordingly, particulate matters corrected by the particulate filter need to be removed for regeneration of the particulate filter to maintain filtering performance for correcting particulate matters in exhaust gas.
A conventional particulate filter has an oxidation catalyst such as platinum to perform regeneration of the particulate filter using oxidizing property of the catalyst. Specifically, fuel is supplied to the particulate filter in a post injection, in which fuel is injected in an exhaust stroke of the engine, for example.
The particulate filter oxidizes particulate matters accumulating on the filter to remove the particulate matters with combustion heat. Thereby, particulate matters, which are hard to be oxidized compared with the injected fuel, can be eliminated from the particulate filter.
When regeneration of the particulate filter is frequently performed, fuel efficiency of the engine is degraded. By contrast, when an interval between regenerations of the particulate filter becomes excessively long, an amount of particulate matters accumulating on the filter becomes excessively large. In this case, when regeneration of the filter is performed, particulate matters on the filter may quickly burn, and temperature of the particulate filter may excessively increase, therefore the particulate filter may be broken. Therefore, the amount of particulate matters is preferably estimated to determine the timing of regeneration of the filter. According to JP-A-7-332065, differential pressure between an inlet of the particulate filter and an outlet of the particulate filter is detected to determine a regeneration timing of the particulate filter. Specifically, when the differential pressure of the particulate filter exceeds a threshold, an amount of particulate matters on the filter is determined to be large, and regeneration of the particulate filter is performed.
However, even when the operating condition of the engine such as the differential pressure of the particulate filter is substantially the same, actual amount of particulate matters accumulating on the particulate filter may vary. Accordingly, it is difficult to precisely evaluate the amount of particulate matters accumulating on the particulate filter in accordance with only the differential pressure of the particulate filter.